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E Heyde

Publications and source records attributed to E Heyde.

At least 19 recordsLinked to original sources
PubMed1983

Chorismate mutase-prephenate dehydrogenase from Escherichia coli: spatial relationship of the mutase and dehydrogenase sites.

The inhibition of the bifunctional enzyme chorismate mutase-prephenate dehydrogenase (4-hydroxyphenylpyruvate synthase) by substrate analogues has been investigated at pH 6.0 with the aim of elucidating the spatial relationship that exists between the sites at which each reaction occurs. Several chorismate and adamantane derivatives, as well as 2-hydroxyphenyl acetate and diethyl malonate, act as linear competitive inhibitors with respect to chorismate in the mutase reaction and with respect to chorismate in the mutase reaction and with respect to prephenate in the dehydrogenase reaction. The similarity of the dissociation constants for the interaction of these compounds with the free enzyme, as determined from the mutase and dehydrogenase reactions, indicates that the reaction of these inhibitors at a single site prevents the binding of both chorismate and prephenate. However, not all the groups on the enzyme, which are responsible for the binding of these two substrates, can be identical. At lower concentrations, citrate or malonate prevents reaction of the enzyme with prephenate, but not with chorismate. Nevertheless, the combining sites for chorismate and prephenate are in such close proximity that the diethyl derivative of malonate prevents the binding of both substrates. The results lead to the proposal that the sites at which chorismate and prephenate react on hydroxyphenylpyruvate synthase share common features and can be considered to overlap.

Binding Sites↗
PubMed1982

Characterization of monofunctional chorismate mutase/prephenate dehydrogenase enzymes obtained via mutagenesis of recombinant plasmids in vitro.

Mutagenesis in vitro has been used to obtain mutant forms of the bifunctional enzyme, chorismate mutase/prephenate dehydrogenase. Plasmid DNA containing the genes that code for the enzyme was treated with hydroxylamine and the resulting products were used to transform strains of Escherichia coli. Two types of mutant were isolated. One contained enzyme which was mutase-positive, dehydrogenase-negative while the other did not exhibit either activity. Kinetic and physical analysis of one of the purified monofunctional enzymes showed that the loss of dehydrogenase activity was due to modification of the binding site for NAD. The results open the way for molecular studies of structure-function relationships with this bifunctional enzyme.

Chemical Phenomena↗
PubMed1979

Chorismate mutase-prephenate dehydrogenase from Aerobacter aerogenes: evidence that the two reactions occur at one active site.

The relationship between the sites for catalysis of two reactions by the bifunctional enzyme chorismate mutase--prephenate dehydrogenase has been investigated. The results are consistent with the occurrence of both reactions at one active site. Comparisons have been made between experimental data for the time course of the overall reaction and computer simulations, according to various models for the relationship between the mutase and dehydrogenase sites. A model based on a single active site is consistent with the time course data if a minor proportion of the chorismate that reacts can be converted through to (hydroxyphenyl)pyruvate without the intermediate release of prephenate. Consistent with this requirement, some channeling of radioactivity from chorismate to (hydroxyphenyl)pyruvate has been detected. A model based on two separate sites has also been considered; the simulations show that if this model applies there is no need to postulate any channeling of the intermediate, prephenate, between the sites and there must be marked inhibition of the dehydrogenase reaction by chorismate. Since channeling has been observed and chorismate increases the dehydrogenase rate under all conditions, the two-site model appears unlikely. Consistent with the one-site model are the observations that a variety of inactivating conditions cause parallel loss of mutase and dehydrogenase activity and that identical protection against inactivation of both mutase and dehydrogenase by iodoacetamide is afforded by prephenate.

Binding Sites↗
PubMed1978

Kinetic studies on the reactions catalyzed by chorismate mutase-prephenate dehydrogenase from Aerobacter aerogenes.

Steady-state kinetic techniques have been used to investigate each of the reactions catalyzed by the bifunctional enzyme, chorismate mutase-prephenate dehydrogenase, from Aerobacter aerogenes. The results of steady-state velocity studies in the absence of products, as well as product and dead-end inhibition studies, suggest that the prephenate dehydrogenase reaction conforms to a rapid equilibrium random mechanism which involes the formation of two dead-end complexes, viz, enzyme-NADH-prephenate and enzyme-NAD+-hydroxyphenylpyruvate. Chorismate functions as an activator of the dehydrogenase while both prephenate and hydroxyphenylpyruvate acted as competitive inhibitors in the mutase reaction. By contrast. bpth NAD+ and NADH function as activators of the mutase. Values of the kinetic parameters associated with the mutase and dehydrogenase reactions have been determined and the results discussed in terms of possible relationships between the catalytic sites for the two reactions. The data appear to be consistent with the enzyme having either a single site at which both reactions occur or two separate sites which possess similar kinetic properties.

Allosteric Regulation↗
PubMed1976

A unifying concept for the active site region in aspartate transcarbamylase.

Recent investigations on the aspartate transcarbamylases (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) of Escherichia coli and Streptococcus faecalis indicate that there is a site on each enzyme, apart from the active site, at which anions can bind. It is suggested in this paper that the location of such an anion binding site on the E. coli enzyme may be directly adjacent to the part of the active site at which carbamyl phosphate binds. This hypothesis is based on data demonstrating a lack of correlation between spectral changes and kinetic effects, and on a new interpretation of results obtained with N-(phosphonacetyl)-L-aspartate, which has previously been considered to act as a transition state analogue. Such a hypothesis could explain other puzzling observations made on the catalytic subunit of this enzyme, including the dependence of substrate inhibition by aspartate on the nature of the second substrate, and the ease of formation of a dead-end enzyme-aspartate-carbamyl-aspartate complex.

Aspartate Carbamoyltransferase↗
PubMed1976

Studies on inosine monophosphate dehydrogenase. An associating-dissociating system.

The techniques of polyacrylamide gel electrophoresis, sedimentation velocity and frontal analysis on Sephadex have been used to demonstrate that preparations of IMP dehydrogenase (IMP: NAD+ oxidoreductase, EC 1.2.1.14) from Aerobacter aerogenes consist of a mixture of molecular weight isomers. Further, it has been shown that dissociation of the higher molecular weight forms is promoted by urea, sodium dodecyl sulphate and dithiothreitol. Under conditions comparable to those used for kinetic analyses, the enzyme has a molecular weight of about 86000 and this is the smallest active species that has been observed. In the absence of a reducing agent, the enzyme undergoes polymerization and is devoid of catalytic activity. From the amino acid composition and peptide map, it appears that the molecule with a molecular weight of 86000 is made up of two identical polypeptide chains.

Amino Acids↗
PubMed1976

Studies on inosine monophosphate dehydrogenase. Steady state kinetics.

The reaction catalyzed by IMP dehydrogenase (IMP: NAD+ oxidoreductase EC 1.2.1.14) from Aerobacter aerogenes has been investigated kinetically at pH 8.1 as a three reactant system by means of steady-state velocity studies in the absence of products, as well as by inhibition studies using products and substrate analogues. The mechanism appears to be a partially random one in which IMP and K+ can bind randomly to the free enzyme while NAD does not react unless K+ or both K+ and IMP are present on the enzyme. While the steady-state velocity data can be analysed adequately on the basis that rapid equilibrium conditions apply, this is only an approximate description of the mechanism since product inhibition studies indicate that there is a significant concentration of an enzyme-XMP (enzyme-K-XMP) complex in the steady-state.

Enterobacter↗
PubMed1976

Studies on inosine monophosphate dehydrogenase. Isotope exchange at equilibrium.

Investigations on the mechanism of the IMP dehydrogenase (IMP: NAD+ oxidoreductase, EC 1.2.1.14) reactions have been made at pH 7.0 by measuring rates of isotope exchange at chemical equilibrium with K+ maintained at a constant concentration. The results are generally in accord with the conclusions reached on the basis of the steady-state kinetic data obtained previously and confirm that there is random addition of IMP and NAD to the enzyme. The data also indicate clearly that at pH 7.0 catalysis is faster than the rate of IMP and/or XMP release which is rate limiting for the reaction sequence. The binding of IMP to the enzyme at pH 8.1 has been demonstrated to occur in the absence of both K+ and NAD and id independent of the K+ concentration.

Carbon Radioisotopes↗